Problems with C++ abstract class (I can do it in Java, but not in C++!)

Question

First of all, I searched this problem and found a lot of similiar questions, but I couldn't find an answer that fixed my problem. I am very sorry if that is just me being dumb.

What I am trying to do is make the constructor of an abstract class call a function that is pure virtual. In Java, this works, because the subclass provides the implementation of the abstract method that is called. However, in C++, I get this linker error:

test.o:test.cpp:(.text$_ZN15MyAbstractClassC2Ev[MyAbstractClass::MyAbstractClass
()]+0x16): undefined reference to `MyAbstractClass::initialize()'
collect2: ld returned 1 exit status

Here is my code:

#include <iostream>

class MyAbstractClass {
protected:
    virtual void initialize() = 0;

public:
    MyAbstractClass() {
        initialize();
    }
};

class MyClass : public MyAbstractClass {
private:
    void initialize() {
        std::cout << "yey!" << std::endl;
    }
};

int main() {
    MyClass *my = new MyClass();
    return 0;
}

As a further explanation of what I am trying to do, here is code in Java that achieves my goal:

public abstract class MyAbstractClass {

    public MyAbstractClass() {
        initialize();
    }

    protected abstract void initialize();
}

public class MyClass extends MyAbstractClass {

    protected void initialize() {
        System.out.println("Yey!");
    }

    public static void main(String[] args) {
        MyClass myClass = new MyClass();
    }
}

This code prints "Yey!". Any help much appreciated!

Answer 1

MyAbstractClass() {
    initialize();
}

That will not perform a virtual dispatch to MyClass::initialize(), because at this stage of the object's construction its MyClass parts haven't been created yet. Thus you really are invoking MyAbstractClass::initialize() and, as such, it must be defined. (Yes, pure virtual member functions can be defined.)

Try to avoid invoking virtual member functions from constructors, because this sort of stuff will happen and catch you out. It rarely makes sense to do it.

Also, try to avoid initialize() functions; you already have constructors to play with.

---

Update

Actually, though you may take the above as read for any other virtual member function, invoking a pure virtual member function from the constructor yields Undefined Behaviour. So don't even try!